Field of the Invention
The present invention relates to a radio-frequency (RF) load control system, and more specifically, to a low-power RF receiver for use in RF control devices, such as, a battery-powered motorized window treatment or a two-wire dimmer switch.
Description of the Related Art
Control systems for controlling electrical loads, such as lights, motorized window treatments, and fans, are known. Such control systems often use the transmission of radio-frequency (RF) signals to provide wireless communication between the control devices of the system. The prior art lighting control systems include wireless remote controls, such as, table-top and wall-mounted master controls (e.g., keypads) and car visor controls. The master controls of the prior art lighting control system each include a plurality of buttons and transmit RF signals to load control devices (such as dimmer switches) to control the intensities of controlled lighting loads. The master controls may also each include one or more visual indicators, e.g., light-emitting diodes (LEDs), for providing feedback to users of the lighting control system. The car visor controls are able to be clipped to the visor of an automobile and include one or more buttons for controlling the lighting loads of the lighting control system. An example of a prior art RF lighting control system is disclosed in commonly-assigned U.S. Pat. No. 5,905,442, issued on May 18, 1999, entitled METHOD AND APPARATUS FOR CONTROLLING AND DETERMINING THE STATUS OF ELECTRICAL DEVICES FROM REMOTE LOCATIONS, the entire disclosure of which is hereby incorporated by reference.
Some of the wireless control devices of the prior art lighting control systems are powered by batteries, which have limited lifetimes that are dependent upon the current drawn from the batteries as well as how often the control devices are used. The RF circuitry (i.e., the transmitters, receivers, or transceivers) of the wireless control devices is one of the primary consumers of battery power in the devices. Therefore, typical prior art battery-powered wireless control devices have attempted to limit the amount of time that the control devices are actively transmitting RF signals. In addition, when the prior art battery-powered wireless control devices are not presently transmitting or receiving RF signals, the RF circuitry is put into a sleep mode in which these circuits drawn less current from the batteries. The RF circuitry is periodically woken up to determine if any RF signals are being received. Thus, the amount of time that the RF circuits are awake as compared to the amount of time that the RF circuits are asleep affects the amount of current drawn from the batteries as well as the lifetime of the batteries.
U.S. Pat. No. 7,869,481, issued Dec. 28, 2010, entitled LOW POWER RF CONTROL SYSTEM, describes a motorized window treatment having an RF receiver, for allowing the motorized window treatment to be controlled from a handheld RF remote control. The remote control transmits command signals that each include a pre-sync pulse time and subsequent message data. To determine if an RF command signal is being transmitted by the RF remote control, the RF receiver of the motorized window treatment periodically wakes up for a short period of time at a rate that ensures that the RF receiver checks for RF signals at least two times during the amount of time required to transmit the pre-sync pulse time of each control signal. For example, if the pre-sync pulse time is 30 milliseconds, the RF receiver wakes up at least two times each 30 milliseconds. Thus, the amount of time that the RF circuitry is awake as compared to being in the sleep mode is dependent upon a characteristic of the command signals, i.e., the length of the pre-sync pulse time, and must be shorter than the pre-sync pulse time. Therefore, the sleep time cannot be increased (to thus decrease the power consumption of the RF receiver) without increasing the pre-sync pulse time, which will decrease the throughput of the system. In addition, the length of the command signals may be limited by national or regional standards.
Therefore, there is a need for a low-power RF receiver that may be used in battery-powered control devices to lead to longer battery lifetimes. Particularly, there is a need for a low-power RF receiver that is able to check for RF signals at a rate that is not limited by a characteristic of each of the transmitted RF signals.